1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a method for fabricating the same, in which a dielectric constant of an insulating film disposed between signal lines is minimized to prevent coupling.
2. Discussion of the Related Art
In general, the liquid crystal display includes two sheets of glass plate disposed opposite to each other and a liquid crystal sealed between the glass plates. A bottom plate has a matrix of data lines and gate lines, and thin film transistors and pixel electrodes disposed at crossing points of the data lines and the gate lines, and a top plate has common electrodes and R, G, B color filter layers. The liquid crystal is injected between the bottom and top plates, which are then inserted between polarizing plates, to make a transmissive liquid crystal display.
In detail, the bottom plate includes a plurality of parallel gate lines spaced apart a certain distance from each other on a transparent substrate, such as glass or quartz, and a plurality of parallel data lines arranged orthogonally to the gate lines, spaced apart a certain distance from each other. Each pixel region has a pixel electrode and a thin film transistor that utilizes the gate line as its gate electrode and the data line as its source electrode for applying a signal on the data line to the pixel electrode in response to a signal on the gate line. That is, as shown in FIG. 1, the bottom plate 1 includes thin film transistors, spaced apart a certain distance from each other, formed thereon each having a gate electrode G connected to the gate line, a source electrode S connected to the data line and a drain electrode D. Each of the pixel regions has a pixel electrode 2a, formed therein, connected to the drain electrode D of the thin film transistor 2. The top plate 3 includes black matrix layers 4 in the form of a net, formed thereon, for blocking the transmission of light, in respective portions, excluding the pixel electrodes 2a formed on the bottom plate 1. R, G, B color filter layers 5 are formed thereon, between the black matrix layers 4. Common electrodes 6 extend to the color filter layers 5 and the black matrix layers 4.
Besides the aforementioned transmissive type liquid crystal display, there is a reflective type liquid crystal display, which utilizes environmental lights, including a light reflective plate of an aluminum membrane attached onto the back of the bottom plate for reflecting incident light out through the top plate for displaying. In modes, such as a TN (Twisted Nematic) or STN (Super Twisted Nematic), the polarizing plate and a checkered plate, forming a reflective plate, are attached to each other and overlap. In general, in a typical application the reflective type is only a monochromic display, and a reflective color display having the color filter applied thereto exhibits a problem in that the display not only is dim, but also has poor color quality.
As illustrated in FIG. 2, the structure of conventional liquid crystal displays suffers from parasitic capacitance between signal lines, such as between a data line and a pixel electrode.
An insulating film 23, which acts as a dielectric, is disposed between a first conductor 21 and a second conductor 22 that overlap each other a distance l. A static capacitance C between the two conductors can be expressed with the following equation. ##EQU1## Where .epsilon..sub.65 is a relative dielectric constant, .epsilon..sub.o is a dielectric constant in vacuum (i.e., .epsilon..sub.o =1), S represents an overlapped area of the two conductors (I x w), and d is a distance between the two conductors.
For example, if the distance d between the two conductors is 2 .mu.m, the width w of the conductor is 2 .mu.m, and an insulating film with a relative dielectric constant .epsilon..sub..gamma.= 3.0 is applied, the static capacitance between the two conductors will be as follows. ##EQU2##
It follows from equations (1) and (2) that to minimize the static capacitance formed between the two conductors, a material having a small relative dielectric constant .epsilon..sub..gamma. should be used, the distance d between the two conductors should be made as large as possible, or the overlapped areas between the two conductors should be made as small as possible.
However, the structure of a conventional liquid crystal display limits the degree of control over these parameters. First, control over the value of the dielectric constant of the insulating film is limited by the choice of suitable materials. Second, when the insulating film is formed of an inorganic material, the problems of deposition rate and pattern formation become critical. If the insulating film is formed of an organic material fabrication problems arise, such as focusing and limitation in the pattern size.